The present invention relates to a device for determining a deterioration condition of an exhaust gas purifier disposed in an exhaust passage in an internal-combustion engine (hereinafter referred to as an “engine”).
An exhaust gas purifier (catalyst) is disposed in an exhaust passage of an engine of a vehicle in order to purify exhaust gas from the engine. The catalyst purifies the exhaust gas by oxidizing or reducing hydrocarbon HC, nitrogen oxide NOx and carbon monoxide CO which are three major pollutants discharged from the engine.
However, as the usage time of the catalyst passes on, an effective surface area of the catalyst gradually decreases, resulting in deterioration of the catalyst. When the deterioration increases, purification of the pollutants does not work properly and a proportion of the pollutants in the exhaust gas emitted to the atmosphere increases, which may exert a bad influence upon the environment.
Therefore, it is important in the purification of the exhaust gas to detect the deterioration condition of the catalyst. There are disclosed some techniques as an approach for diagnosing such deterioration condition of the catalyst. The National Entry Publication (Kohyo) No. 2001-522015 discloses a technique for evaluating efficiency of a catalytic converter by changing an air-fuel ratio vibrationally and using an output signal of an oxygen sensor. The Japanese Patent Application Unexamined Publication (Kokai) No. HEI7-269330 discloses a technique for evaluating responsiveness of a catalyst based on a response time of an exhaust gas sensor disposed downstream of the catalyst when a step-wise change is given to fuel by a fuel-cut or the like.
The Japanese Patent No. 2938288 discloses a technique for determining deterioration of a catalyst based on a result of comparison between an output of an air-fuel ratio sensor disposed downstream of the catalyst and a filtered value of the output of the air-fuel ratio sensor. Besides, the Japanese Patent No. 3216067 discloses a technique for determining deterioration of a catalyst based on a phase difference in respective sensor values on the upstream side and downstream side of the catalyst.
The technique disclosed in Kokai No. 2001-522015 determines a capability of the catalyst based on an attenuation of the high-frequency components in the remaining signal of the sensor output from which the filtered output is subtracted. However, this technique has a problem of poor accuracy because it determines the catalyst capability based on only the positive value signal of the remaining signal. At present, a need for detecting a deterioration condition of an exhaust gas purifier more precisely is increasing in order to cope with reinforcement of the emission control.
Besides, the technique disclosed in the Kokai No. HEI7-269330 determines a deterioration condition of the catalyst based on a response time when the step-wise change is applied to the fuel. However, the response time may change depending on the condition of the catalyst such as the amount of O2 stored in the catalyst. Furthermore, the response time may change depending on the influence of the A/F ratio spike due to the transient operation.
Thus, it is an objective of the present invention to provide a catalyst evaluation technique for evaluating presence/absence of deterioration of the catalyst more precisely with little influence by a catalyst condition at the evaluation time.